1) Field of the Invention
This invention relates to satellite up-link fade control.
2) Description of the Related Art
In a digital data transmission channel link, particularly a satellite transmission channel link, it is known for modulation techniques to use symbols arranged as points in a particular constellation pattern to represent digital data. The constellation shows all possible combinations of complex (I and Q) samples of the data being transmitted and the constellation pattern is an overlay of all possible positions of each data sample at a particular point. Thus, for each symbol, we may have four sampling points. Typical techniques are those of phase shift keying (PSK) and quadrature amplitude modulation (QAM). Common techniques are quadrature phase shift keying (QPSK) which is used for digital satellite transmission for consumer TV applications, and 8 PSK which is used, for example, for satellite news gathering applications. It is a desire to utilise higher order modulation methods such as 16 PSK and 16 QAM to permit transmission at a higher bit rate so as to facilitate a greater number of channels to be carried within a predefined bandwidth of a particular transmission link.
For 16 QAM operation, the highest drive level which can be used practically for a peak power limited satellite channel is that which forces the satellite transponder to saturation at the corners of the 16 QAM constellation, and such a constellation diagram is shown in FIG. 1 having axes I and Q. It is desirable to allow the satellite transponder to operate at, or very close to, saturation and such operation is known as “corners at saturation” operation.
As is well known, transmission of a modulated signal through a transmission channel such as a terrestrial link, cable or satellite results in distortion of the signal. The distortion is due, at least in part, to non-linear effects upon a signal as it passes through the transmission link. The distortion, in terms of magnitude and/or phase, results in a change in location of the constellation points for any given modulation scheme and an increase in the order of modulation results in a decrease in the distance between constellation points, thereby leading to distortion having a greater effect. Such distortion has the disadvantage of producing errors in demodulation.
It is known to compensate for such non-linear distortion effects within transmission links by use of a pre-correction compensator. Signal pre-distortion performed at radio frequencies (RF), intermediate frequencies (IF) or base band frequencies is often carried out by application of an inverse function of the distortion to be expected of the signal in the transmission path. Such pre-distortion is disclosed in WO-A-95132561 and U.S. Pat. No. 4,992,754. Such forms of pre-correction tend to generate out-of-band components which are passed through to amplifiers in the transmission channel. Where the amplifier has an input filter, as is common for amplifiers used in satellite transmission links, then these out-of-band components are usually filtered out prior to amplification. Thus, the input signal to the amplifier is not the entire signal. This means that pre-correction is not effective for correction of amplifiers contained within satellite transponders where the bandwidth of the incoming signal is high in relation to the bandwidth of the transponder. Further, for higher order modulation schemes, such a form of pre-correction requires very high clocking rates in order to generate the wide-band pre-distortion components.
The foregoing problems are at least partially mitigated by the apparatus disclosed in WO-A-0025495, which discloses an arrangement for pre-distorting a signal so as to offset later distortion of the signal during transmission across a satellite transmission link which contains root Nyquist bandpass filters in respective up and down links. The apparatus includes a plurality of identical pre-distorting stages each of which generates an approximation of the required pre-distortion. Each successive stage receives an approximation from the preceding stage so that errors in successive approximations converge towards zero with increase in the number of stages.
The use of dynamic pre-correction requires that the operating point of the transmitter high power amplifier and of the satellite transponder is stable because the required correction is dependent upon the operating point. Some satellite transponders include automatic gain control (AGC) which maintains a constant drive level to the satellite power amplifier irrespective of the signal level at the input to the satellite. As a result, the only effect of an up-link fade is that the up-link carrier to noise ratio is worsened. However, many satellites do not have AGC and the present invention seeks to provide an up-link fade control system for satellite transponders not having AGC.